Fix README not to refer moved file

`cargo run tests/snippets/whats_left_to_implement.py`
-> `./whats_left.sh`

Author: youknowone

README.md

@@ -85,7 +85,7 @@ Another approach is to checkout the source code: builtin functions and object me
 and easiest way to contribute.
 
 You can also simply run
-`cargo run tests/snippets/whats_left_to_implement.py` to assist in finding any
+`./whats_left.sh` to assist in finding any
 unimplemented method.
 
 # Testing